The present invention relates to a photoelectric converter for receiving light impinging on an original image and reflected therefrom and for converting the light into an electric signal. In particular, the present invention relates to a linear image sensor suitable for an image reading apparatus such as a facsimile and an image scanner.
FIG. 6 shows a circuit diagram of an image sensor IC used in a conventional image reading apparatus, and FIG. 7 shows a timing chart. The image sensor is described in Japanese Patent Application Laid-open No. Hei 10-051164.
An N-type region of a photodiode 101 is connected to a positive power source voltage terminal VDD, and a P-type region of the photodiode 101 is connected to a drain of a reset switch 102 and a gate of a source follower amplifier 103. A source of the reset switch 102 is supplied with a reference voltage VREF1. A source of the source follower amplifier 103, which is an output terminal, is connected to a reading switch 105 and a constant current source 104. A gate of the constant current source 104 is supplied with a constant voltage of a reference voltage VREFA, the drain is grounded, and the source is connected with VDD. The drain of the reading switch 105 is connected with the source of the source follower amplifier 103 and the source of the reading switch 105 is connected with a common signal line 106. Elements in a frame of a photoelectric conversion block An shown in FIG. 6 are provided in a number corresponding to pixels. The photoelectric block An represents a photoelectric block in an n-th bit.
The common signal line 106 is input to an inversion terminal of an operational amplifier 109 through a resistor 110 and is grounded by a capacitor 108. An output terminal of the operational amplifier 109 is connected to an output terminal 116 through a chip selecting switch 112 and a capacitor 113. The common signal line 106 is connected to a signal line reset switch 107, and a source of the signal line reset switch 107 is supplied with a reference voltage VREF2. A resistor 111 is connected between an output terminal of the operational amplifier 109 and the inversion terminal thereof. A non-inversion terminal of the operational amplifier 109 is fixed at a constant voltage VREF3. The operational amplifier 109, and the resistors 110 and 111 constitute an inversion amplifier D.
An output terminal 116 of an image sensor is connected to a drain of a MOS transistor 114, and a source of the MOS transistor 114 is supplied with a reference voltage VREF4. The output terminal 116 of the image sensor is also connected to a capacitor 115 such as a parasitic capacitor. The capacitors 113 and 115, and the MOS transistor 114 constitute a clamping circuit C.
In the image sensor with the above-mentioned configuration, after light is reflected from the original image and received, electric charge accumulates on the photodiode, producing a light signal voltage. The light signal voltage is then read from the photodiode and the photodiode is reset. In another step, a reference voltage is read, and a difference between the light signal voltage and the reference voltage is obtained. Therefore, reset noise contained in the reference voltage is different from that in the light signal voltage. More specifically, reset noise at different timings is compared, so that random noise is rather large. Furthermore, since the read of a reference voltage, reset of a photodiode, and read of a light signal voltage are conducted successively in each bit, high-speed read is difficult.
Therefore, with the foregoing in mind, it is an object of the present invention to provide a photoelectric converter that realizes high-speed read while reducing random noise.
In order to solve the above-mentioned problem of the prior art, a photoelectric converter includes charge transfer circuit between an output terminal of photoelectric converter and an input terminal of an amplifier, the input terminal of the amplifier being connected to a reset circuit, wherein after accumulation of a light signal of the photoelectric converter, a reference signal held at the input terminal of the amplifier is read from the output terminal of the amplifier, the charge transfer circuit is turned on to transfer light signal charge of the photoelectric converter to the input terminal of the amplifier, after the charge transfer circuit is turned off, a light signal held at the input terminal of the amplifier is read from the output terminal of the amplifier as a light signal, the charge transfer circuit and the reset circuit are turned on to reset the output terminal of the photoelectric converter and the input terminal of the amplifier, and after the reset circuit is turned off, the charge transfer circuit is turned off, whereby subsequent accumulation of a light signal is conducted.
Alternatively, a photoelectric converter includes charge transfer circuit between an output terminal of photoelectric converter and an input terminal of amplifier, the output terminal of the photoelectric converter being connected to a reset circuit, wherein after accumulation of a light signal of the photoelectric converter, a reference signal held at the input terminal of the amplifier is read from the output terminal of the amplifier, the charge transfer circuit is turned on to transfer light signal charge of the photoelectric converter to the input terminal of the amplifier, after the charge transfer circuit is turned off, a light signal held at the input terminal of the amplifier is read from the output terminal of the amplifier as a light signal, the charge transfer circuit and the reset circuit are turned on to reset the output terminal of the photoelectric converter and the input terminal of the amplifier, and after the reset means is turned off, the charge transfer circuit is turned off, whereby subsequent accumulation of a light signal is conducted.
Alternatively, there is provided a photoelectric converter for outputting a reference signal and a light signal from a photoelectric converting portion through an amplifier, wherein the reference signal is transferred to reference signal holding circuit through reference signal transfer circuit, the light signal is transferred to light signal holding circuit through light signal transfer circuit, the reference signal holding circuit is connected to an input terminal of a second amplifier through second reference signal transfer circuit, the light signal holding circuit is connected to an input terminal of the second amplifier through second light signal transfer circuit, during a signal reading period, the second light signal transfer circuit is turned onto transfer the light signal held at the light signal holding circuit to the input terminal of the second amplifier, a light signal output is read from an output terminal of the second amplifier, after the second light signal transfer circuit is turned off or at the same time when the second light signal transfer circuit is turned off, the second reference signal transfer circuit is turned on to transfer the reference signal held at the reference signal holding circuit to the input terminal of the second amplifier, and a reference signal output is read from the output terminal of the second amplifier.
According to the above-mentioned reading method, a reference voltage and a light signal voltage containing the same OFF noise of a reset switch are read successively. Therefore, if a difference between the voltages is amplified, a photoelectric converter in which not only fixed pattern noise but also random noise is small can be obtained. Furthermore, since a reference voltage and a light signal voltage can be temporary read to separate capacitors, simultaneously with respect to all the bits, this operation can be made slower. Thus, the area of a circuit to be read can be decreased. Furthermore, a light signal voltage and a reference voltage are read from the capacitor successively in the order of bits through a source follower amplifier. Therefore, high-speed read can be conducted without a reset period.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.